PhoneGap is a platform for building mobile application using Web technologies: HTML, CSS and Javascript. Mobile platforms like Android, iPhone or Blackberry are not ready yet to manage such application, that is why PhoneGap is filling in the gap by providing a plugin system to use native call through Javascript.
This is very interesting platform because there is a global movement between Web technologies interacting with mobile, smartphone, … You know: HTML5, CSS3, Boot 2 Gecko (see also: B2G Wiki), Apache Cordova, SOCIETIES project, …